Air is separated in air separation plants that employ cryogenic rectification to separate the air into products that include nitrogen, oxygen and argon. In such plants, the air is compressed, purified of higher boiling contaminants such as carbon dioxide and water, cooled to a temperature suitable for the distillation of the air and then introduced into a distillation column system.
In one typical distillation column system, the air is separated in a higher pressure column into a nitrogen-rich vapor column overhead and a crude liquid oxygen column bottoms, also known as kettle liquid. A stream of the crude liquid oxygen column bottoms is introduced into a lower pressure column for further refinement into an oxygen-rich liquid column bottoms and a nitrogen-rich vapor column overhead. The lower pressure column operates at a lower pressure than the higher pressure column and is thermally linked to the higher pressure column by a heat exchanger known as a condenser reboiler. The condenser reboiler condenses a stream of the of the nitrogen-rich vapor column overhead through indirect heat exchange with the oxygen-rich liquid column bottoms to produce liquid nitrogen reflux for both the higher and lower pressure columns and to create boilup in the lower pressure column by vaporization of part of the oxygen-rich liquid column bottoms produced in such column.
In any type of air separation plant, liquid and vapor that can be composed of nitrogen-rich and oxygen-rich liquid and vapor are introduced into a main heat exchanger and passed in indirect heat exchange with the incoming air to help cool the air and to be taken as products from the warm end of the main heat exchanger. In addition, liquid products enriched in oxygen, nitrogen or both can be taken from the distillation column system as liquid products. Also, all or a portion of liquid streams removed from columns can be pumped to produce a pumped or pressurized liquid which is heated in the main heat exchanger or a separate heat exchanger designed to operate at high pressure and produce a enriched products as either a vapor or a supercritical fluid.
Since an air separation plant must be maintained at cryogenic temperatures in order to allow the air to be distilled, refrigeration must be imparted to the plant in order to compensate for heat leakage into the plant and warm end losses from the main heat exchanger or other heat exchanger operated in association therewith. Further, the removal of liquid products will also remove imparted refrigeration that must also be compensated through introduction of refrigeration into the plant. This is commonly done by forming a compressed air stream by introducing the compressed and purified air into a booster compressor. The compressed air stream after such further compression is then introduced, either directly or after partially cooling such stream, into a turbo-expander to produce an exhaust stream that is introduced into the distillation column system. In this regard, such exhaust stream can be introduced into the lower pressure column or the higher pressure column.
In large part, the ongoing expense in operating an air separation plant is the cost of electricity that is consumed in compressing the air. As mentioned above, when liquid is to be taken as a product, further compression will be required to generate the refrigeration that will be required when such liquid products are produced. However, the demand for liquid products and the cost of electricity are not constant. For instance, the cost of electricity and the liquid demand will often be less during evening hours as compared with daylight electricity costs and liquid demands. Consequently, air separation plants can be designed to cyclically produce a greater share of liquid products or higher pressure products when electricity is less expensive.
Many air separation plants also have a need to vary the pressure of the gaseous and liquid products produced. Examples may include an air separation plant that feeds multiple pipelines or dual air separation plant that is specifically designed having dual cores or dual cold boxes to produce products at different pressures. In such situations, there is occasionally the need to alter the product mix requiring a switch or reallocation to or from the higher pressure product or higher pressure pipeline. Yet another common scenario is a dual or single pressure air separation plant that selectively modifies the product slate to produce more argon or low pressure nitrogen when electricity is less expensive in lieu of high pressure or medium pressure oxygen.
The conventional solution or technique used to achieve this variation in product pressures is to adjust the compressor guide vanes to reduce BAC pressure. However, when lowering the product pressures, the conventional solution of varying the compressor guide vanes to reduce BAC pressure often leads to little or no power savings and thus no significant cost reductions. As will be discussed, the present invention provides a method of separating air and an air separation plant which among other advantages, allows a booster compressor to by bypassed to turn down or turn up the pressurized product pressures and/or production rates with greater efficiencies and cost savings than are contemplated in the prior art.